Biosorption of Heavy Metal Ions by Microalgae: Mechanisms and Conditioning

Abstract

Wastewater contaminated with heavy metal ions (HMIs), stemming from human activities and natural disasters, poses substantial threats to both the environment and human health. The unchecked release of untreated wastewater into natural water bodies leads to severe pollution, upsetting ecological balance. To address this pressing challenge, microalgae-based biosorption technology has emerged as a promising solution for the efficient removal of HMIs from wastewater. Microalgae, with their extensive surface area and intricate cell wall structures, exhibit remarkable efficacy in HMIs biosorption. This thesis aims at elucidating the fundamental principles governing the interactions between HMI and microalgal cells to help enhance the biosorption capacity of HMI by microalgae from two perspectives: 1) conditioning of biomass by either optimizing the cultivation conditions or downstream processing; and 2) conditioning of the biosorption process for optimal performance of given algal biomass. It was demonstrated that among the tested cultivation conditions, i.e., culture pH, phosphate concentration, nitrate concentration, and dissolved inorganic carbon (DIC) conditions, which all have significant impacts on cell surface structure and therefore biosorption of HMI, DIC is the most significant factor. Furthermore, it was demonstrated that downstream processing of biomass such as lipid extraction with sonication for cell disruption could help enhance specific surface area and removal of lipids from cell wall surfaces, resulting in remarkably elevated HMI biosorption capacities. As for research on biosorption mechanisms, a correlation between HMI properties, i.e., ionic radius and electronegativity, and their biosorption capacities onto certain microalgal biomass, was established, which was validated with both experimental data and literature data. Furthermore, systematic studies on biosorption kinetics, isotherm, and thermodynamics, as well as cell surface characterization, and determination of HMI intracellular and extracellular contents of cells after biosorption were carried out, which converged on the conclusion that biosorption was predominantly monolayer surface adsorption. A mathematical model was proposed and validated, which is a rigid model accounting for the effects of cell size and HMI radius only. Analysis of model differentiation from experimental data led to the hypothesis that the nanostructures on cells, mostly like pili, were the major locations where binding sites for HMI were housed. This research represents a significant step towards ensuring the responsible and sustainable use of microalgae for environmental engineering, promising a cleaner and healthier future.